20/04/2016 7:32 AM AEST

This Is What Feeling Hungry Does To Your Brain

Research reveals exactly why you shouldn't go grocery shopping when you're hungry.

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Scientists find that hunger changes sensory perception in a way that can affect our behavior. 

When you're really hungry, the smell and sight of food takes on a whole new meaning -- so it's unsurprising that going grocery shopping when famished can lead to an overflowing cart and, likely, some buyer's remorse.

A new study on larval zebrafish, to be published in next month's issue of the journal Neuron, shows that we may find it more difficult to resist tasty treats when our stomachs are grumbling because hunger may heighten our sensory perceptions of food. 

Researchers at the Max Planck Institute for Neurobiology in Germany used computer animation to allow different sized dots to move through the visual fields of the zebrafish. Normally, the fish chase small moving dots and swim away from large moving dots, while they avoid and approach mid-sized dots with equal frequency.

As expected, the team noticed that the fish changed their behavior when they were hungry. The hungry fish tended to chase the mid-sized dots significantly more than the satiated fish did.

"We interpret this behavior such that the hungry animals are taking more 'risks' -- after all, the big dots could be bigger fish that are out to eat them," Dr. Herwig Baier, a neuroscientist at the institute and the study's lead author, told The Huffington Post in an email. "What was really interesting was that the brain's visual centers also responded differently to prey-like stimuli."

In hungry fish, Baier explained, a greater number of nerve cells became active in response to the small and mid-sized dots, which suggests that hunger changes the way that the outside world is represented in the brain.

By increasing nerve activity involving sensory perception in the tectum -- a brain region that links visual inputs to behavioral outputs -- hunger likely changes the way that the fish perceive the outside world, making objects appear edible.

The researchers concluded that hunger may increase nerve cell activity in the tectum via two molecular pathways -- the inhibition of hormones that signal low energy levels, and increased activity of the neurotransmitter serotonin, which governs functions like mood, appetite and digestion.

The change of perception directly influences the fish's behavior, making them more likely to go after "risky" food targets than satiated fish. 

In humans, it's likely that something similar happens, with hunger triggering a shift in brain activity involved in sensory perception. The way we classify objects is likely to be significantly affected by how hungry we are -- which may explain why food can be so difficult to ignore when you haven't eaten in a while. 

"We all know that smells and sights of food become more salient when we're hungry," Baier said. "Subconsciously, our visual sense may be better at noticing, for example, ads for fast food restaurants."

But, why study larval zebrafish as a model for the human brain? According to Baier, the tiny vertebrates are actually a surprisingly good way to study the neural bases of behavior. The zebrafish have a similar brain structure to humans and they're transparent, so scientists can use staining methods to clearly observe changes in nerve cell activity. 

"We can image brain activity with cell resolution in a living, intact animal that is watching a movie on a miniature TV screen," Baier said. "We use genetic engineering to make nerve cells of the fish produce fluorescent proteins that light up when the cell is active."

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